1. Field of the Invention
The invention relates to a clock range setting device. More particularly, the invention relates to a clock range setting device capable of avoiding the erroneous bit stream due to the timing drift or the undefined number of lead clock.
2. Description of Related Art
The technology of framing the lead clock is used to expect the timing when the lead clock occurs according to the timing control signal and the text specification used in the TV broadcasting system. FIG. 1 is a schematic timing diagram of the composite teletext video signal. As shown in FIG. 1, the composite teletext video signal, for example, includes a horizontal synchronizing pulse HSY11, a color synchronous pulse CSY11, a lead clock CLK11, and a bit stream BST11. According to the enhanced teletext specification defined by the ETSI EN 300 706, for the time specification of the lead clock CLK11, the lead clock CLK11 is defined as the period from the falling edge of the horizontal synchronizing pulse HSY11 to the second last peak of the lead clock, which is about 12−1.0+0.4 microseconds (us).
Generally, the receiving end of the TV broadcasting system records the number of the sample points of the composite teletext video signal according to the counter value VAL outputted by a counter. Accordingly, along with the increasing of the counter value VAL, the receiving end frames a lead clock range AR11 by setting the initial value and the final value and comparing the counter value VAL. Next, the phase locked loop (PLL) inside the receiving end locks the lead clock CLK11 in the lead clock range AR11, and the output clock of the PLL can serve as the sample clock of the bit stream BST11 later.
However, when being broadcasted in practice, the composite teletext video signal will have an offset relative to the teletext specification in response to the timing drift. FIG. 2A and FIG. 2B are schematic timing diagrams of the composite teletext video signal in response to the timing drift. For example, as shown in FIG. 2A, when the period from the falling edge of the horizontal synchronizing pulse to the second last peak of the lead clock is smaller than 11 us due to the composite teletext video signal in response to the timing drift, the lead clock range framed by the system will include the bit stream, thereby causing the erroneous synchrotron of the PLL and the erroneous bit stream. On the contrary, as shown in FIG. 2B, when the period from the falling edge of the horizontal synchronizing pulse to the second last peak of the lead clock is larger than 12.4 us due to the composite teletext video signal in response to the timing drift, the lead clock range framed by the system will simply include the lead clock. In such a case, the PLL can not be synchronous with the lead clock, thereby causing the erroneous sampling of the bit stream.
Moreover, the teletext specification has not defined a clear number of the lead clock, so that the number of the lead clock may be from 4 to 8 in an actual TV broadcasting system. In this case, as shown in FIG. 3, which is another schematic timing diagram of the composite teletext video signal, the system can not previously obtain the number of the lead clock set in each channel, so that the receiving end will frame the lead clock range by the initially set number of the lead clock, e.g. 7. Accordingly, if the number of the lead clock used in the current TV broadcasting system is 5, it will cause the erroneous synchrotron of the PLL and the erroneous bit stream.